[contrib] Allow more space for Bochs option ROM

Signed-off-by: Michael Brown <mcb30@ipxe.org>

contrib/vm/bochsrc.txt

 #optromimage2: file=optionalrom.bin, address=0xd1000
 #optromimage3: file=optionalrom.bin, address=0xd2000
 #optromimage4: file=optionalrom.bin, address=0xd3000
-optromimage1: file=../../src/bin/intel.rom, address=0xd0000
+optromimage1: file=../../src/bin/intel.rom, address=0xcb000
 
 #optramimage1: file=/path/file1.img, address=0x0010000
 #optramimage2: file=/path/file2.img, address=0x0020000